In some recently developed disk apparatuses to which iterative decoding is applied, a low density parity check (LDPC) code is used as a parity check code added to data written to a disk. In these disk apparatuses, each bit of data read from the disk is decoded by an LDPC decoder based on a log likelihood ratio (LLR) output by a soft-decision maximum-likelihood decoder. A soft output Viterbi algorithm (SOVA) is applied to the soft-decision maximum-likelihood decoder.
LLR is a logarithmic form of the ratio of the probability (likelihood) that the corresponding bit is “1” to the probability that the bit is “0”. When positive, LLR indicates that the corresponding bit is more likely to be “1”. When negative, LLR indicates that the corresponding bit is more likely to be “0”. When LLR is zero, the corresponding bit can be determined to be “1” or “0” at the same probability. That is, when LLR is zero, the corresponding bit is least reliable. Thus, LLR is reliability (probability) data indicative of the degree of reliability (probability) at which the corresponding bit is “1” or “0”.
The LDPC decoder carries out parity check based on LDPC code added to data corresponding to LLRs output by the soft-decision maximum-likelihood decoder, to update LLRs. Based on updated LLRs, the soft-decision maximum-likelihood decoder outputs new LLRs. Thus, LLRs are repeatedly propagated between the soft-decision maximum-likelihood decoder and the LDPC decoder under a predetermined condition. The propagation of LLRs is called probability propagation. Data is decoded by iteration of propagation of LLRs, that is, iterative decoding.
It is assumed that LLRs are partly low in a certain portion of the data because of a defect on a disk. The presence of such a portion may affect the other, higher LLRs owing to probability propagation.
Thus, for example, Jpn. Pat. Appln. KOKAI Publication No. 2008-112527 (hereinafter referred to as the prior art document) discloses a technique to mask the part of LLRs corresponding to a defective portion (hereinafter referred to as a medium defective portion) on the disk based on the above-described LLRs or the amplitude of a signal (read signal) read from the disk by a head. The technique described in the prior art document uses a scaling factor α to reduce the part of LLRs corresponding to the medium defective portion, thus suppressing the adverse effect of propagation of the part of LLRs corresponding to the medium defective portion.
The medium defective portion is roughly classified into a medium defective portion associated with a sharp decrease in the amplitude of the read signal (this type is hereinafter referred to as a first medium defective portion) and a medium defective portion associated with a gradual decrease in the amplitude of the read signal (this type is hereinafter referred to as a second medium defective portion). At the boundary of the first medium defective portion, the amplitude of the read signal or LLRs decrease or increase rapidly. Thus, the boundary of the first medium defective portion can be accurately detected based on the amplitude of the read signal (for example, the moving average of the amplitude) or LLRs.
In contrast, it is difficult to accurately detect the boundary of the second medium defective portion based on the amplitude of the read signal or LLRs. Furthermore, even if the boundary of the second medium defective portion is successfully detected, data is not always successfully read from a data sector with the second medium defective portion. Thus, there has been a demand to effectively control the adverse effect of the second medium defective portion on normal portions.